Apparatus And Method For Flattening Gain Profile Of An Optical Amplifier

ABSTRACT

A change in loading conditions of fiber amplifiers in an optical communications network causes rapid variations in the gain profile of the amplifiers due to spectral hole burning and stimulated Raman scattering. An apparatus for reducing such gain profile variations is described which monitors optical signal perturbations and reacts by adjusting pump powers of the amplifiers and, or fast variable optical attenuator according to a predetermined function stored in the form of constants in controller&#39;s memory. The optical signal is monitored as total power, and the power of light after passing through one or more optical filters. The light detection is relatively fast, whereby the gain profile variations are compensated by fast controlled variable optical attenuator and pump power adjustment upon the change in loading conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. provisional patentapplication No. 60/978,253, filed Oct. 8, 2007, which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention is related to optical fiber amplifiers, and inparticular to flattening the gain profile of erbium doped fiberamplifiers (EDFA), Raman Amplifiers (RA), and hybrid EDFA-RA amplifiers.

BACKGROUND OF THE INVENTION

In a wavelength division multiplexing optical transmission system,various information channels are encoded into light at differentwavelengths, which is combined using a multiplexor. The combined lightis transmitted through an optical fiber and, or an optical fiber networkto a receiver end of the optical fiber. At the receiver end, the signalis separated, or demultiplexed, back into the individual opticalchannels through a de-multiplexor, whereby each optical channel can bedetected by an optical detector such as a photodiode, and theinformation can be reconstructed, channel by channel.

While propagating through the optical fiber, light tends to lose powerdue to the losses related to the physics of how the light interacts withthe optical fiber. Yet some minimal level of optical channel power isrequired at the receiver end in order to decode information encoded inthe optical channel. In order to boost the optical signal propagating inthe optical fiber, optical amplifiers are deployed at multiplelocations, known as nodes, along the transmission link. The opticalamplifiers extend the maximum possible length of the link, in someinstances, from a few hundred kilometers to several thousand kilometers,whereby after each fiber span, the optical signal is amplified to powerlevels close to the original levels of the transmitter. Unfortunately,during the amplification process some amount of noise is introduced intothe optical signal which effectively limits the amount of opticalamplifiers a transmission link can have.

Modern optical communication systems employ erbium doped fiberamplifiers (EDFAs), Raman Amplifiers (RAs) and hybrid EDFA-RAs as meansto boost the optical signal power and thus to extend the communicationsystem reach. Nowadays, optical communication systems have become moreagile and reconfigurable. Reconfiguration of the optical communicationsystem leads to variation of the signal load at the input of theamplifier. At the same time, the goal of the amplifier is to provideconstant gain, which should not depend on the power or wavelengthloading condition; otherwise, some channels will not have sufficientpower and signal-to-noise level at the receiver end, resulting ininformation being lost.

The control electronics of EDFAs partially solves the problem of thevariable signal load. More particularly, the total optical power at theinput and at the output of the amplifier is measured, and the averageoptical signal gain of the amplifier is calculated. The amplifiercontrol electronic circuitry adjusts the amplifier's pump powers througha feedback loop in such a way that the measured optical gain equals tothe desired or “set” optical gain and is not varied significantly intime.

However, it is desired not only to have average gain of the amplifier tobe constant, but also to have the gain of the individual channelconstant and independent from the other channels' presence or absence,that is, independent from the channel load. At the same time, due to thespectroscopy of the erbium doped fiber, namely due to the spectral holeburning (SHB) effect, the gain shape of EDFA does depend on the inputload. Hence even if the average gain of an EDFA is held constant, thegain of the individual channels will vary, leading to undesirableeffects, such as increased bit error rate of the transmission system.

One way in which to address the problem is to check the channel powersat a location in the transmission system, using an optical channelmonitor (OCM). The collected information is then used by the systemcontrol circuitry to adjust a dynamic gain equalizer (DGE) in thetransmission link in such a way that the transmitted spectrum isflattened. The OCM and DGE need not necessarily be at a same location inthe system. The advantage of this approach that it compensates for allgain change inducing impairments of the system, such as stimulated Ramanscattering (SRS) induced tilt, not only EDFA SHB.

However the above approach has several disadvantages. First, because theDGE and OCM are expensive components, they are not generally installedat each amplifier node, thus they compensate several amplifiers at once,which is not optimal. Second, both OCM and DGE are comparatively slowdevices, and thus the correction usually takes a few seconds. This isundesirable for agile communication systems where a typical requirementfor the adjustment for a transient event such as a change of the channelload is on the order of 100 μs, which is 10,000 times shorter than for aDGE/OCM approach.

To address the disadvantage of this compensating technique it has beensuggested by Zhou et al. in an article entitled “Fast control ofinter-channel SRS and residual EDFA transients using amultiple-wavelength forward-pumped discrete Raman amplifier”, OMN4, OFC2007, which is incorporated herein by reference, to measure channelpowers of a limited number of channels that are located at specificwavelengths, 1528.6 nm, 1544.4 nm, and 1559.6 nm in the publishedexample. Subsequently, the Raman pump powers of the Raman amplifier areadjusted using linear feed-forward control. The work is based on RAshaving 3 different wavelengths of Raman pumps. Again, similar to theaforementioned DGE/OCM approach, this compensates not only EDFA SHB, butSRS tilt as well.

The main disadvantage of this technique is the requirement of theconstant presence of those three channels the power of which isconstantly monitored. This is a very limiting requirement for modernagile communication systems. Another potential disadvantage is therequirement to have three additional detectors. Finally, relatively goodSHB compensation is possible only in the presence of three Ramanpumps—the reduction of number of pumps will lead to the reduction of theamount of compensation.

Further, in U.S. Pat. No. 7,359,112 by Nishihara et al. which isincorporated herein by reference, a control apparatus is described whichadjusts the gain of an EDFA based on an amount of wavelengths which iscalculated on the basis of optical power measured in two or threeseparate spectral bands by dedicated photodetectors. One disadvantage ofthis approach is that only one control parameter, specifically the EDFAgain, is adjusted which limits the degree to which both the SHB and SRScan be compensated. Another disadvantage stems from the fact thatcertain load change patterns, for example the patterns which leave thetotal optical power measured in a single spectral band unchanged, willnot be detected by the apparatus of Nishihara et al. and therefore willnot be compensated for by said apparatus.

It is an object of the present invention to provide an apparatus andmethod for flattening a gain profile of an optical amplifier suitablefor suppression of sub-millisecond scale transient variations of gaincaused by changes in the amplifier load which would not requirededicated spectral channels in order to monitor the gain profile. Thisinvention extends the technique that was suggested by Bolshtyansky etal. in an article entitled “Dynamic Compensation of Raman Tilt in aFiber Link by EDFA during Transient Events”, JThA15, OFC 2007, whereinstead of measuring the actual gain change, the device measures someproperty of the transmitted signal, and adjusts the gain profile basedon the measured property of the signal.

SUMMARY OF THE INVENTION

The apparatus of the present invention branches off a small portion of atransmitted optical signal, splits this portion into a plurality ofsub-portions, passes the sub-portions through a set of characteristicoptical filters, and measures the resulting optical powers. Based on themeasurements, the apparatus adjusts the pump power of an erbium dopedfiber amplifier (EDFA) and, or the pump power(s) of a Raman amplifier(RA), and, or the attenuation setting of a fast variable opticalattenuator, according to a pre-defined set of response functions chosento increase the flatness of the gain profile of a hybrid EDFA-RA opticalamplifier.

Thus, in accordance with the invention there is provided an apparatusfor flattening a gain profile G(λ) of a hybrid amplifier comprising anerbium doped fiber amplifier and a Raman amplifier for amplifying astream of optical signals, the apparatus comprising:

a detection device arranged to receive a tapped portion of the stream ofoptical signals in the form of N+1 sub-portions and to provide N+1output signals P₀ . . . P_(N) in dependence upon said tapped portion,wherein N is an integer positive number, the detection devicecomprising: N spectral filters having respective transmission functionsF₁(λ) . . . F_(N)(λ); and N+1 photodetectors for producing the N+1output signals P₀ . . . P_(N) in response to a light impinging thereon,wherein the first sub-portion of the tapped portion of the stream ofoptical signals is coupled to the first photodetector for producing thesignal P₀, and each one of remaining N of said sub-portions of thetapped portion of the stream of optical signals is coupled to one of theN spectral filters coupled to one of the remaining N photodetectors forproducing the signals P₁ . . . P_(N);

a controller arranged to receive said signals P₀ . . . P_(N) from thedetection device and suitably programmed to provide M control signals x₁. . . x_(M) in dependence upon said signals P₀ . . . P_(N), wherein M isan integer positive number and x_(m)=f_(m)(C_(k),P₀ . . . P_(N)),wherein f_(m) is a pre-determined function and C_(k) are pre-determinedconstants, for each m=1 . . . M; and

M spectral actuators S₁ . . . S_(M) arranged to receive said controlsignals x₁ . . . x_(M), respectively, and modify the gain profile G(λ)by a value ΔG(λ) according to the equation

${{\Delta \; {G(\lambda)}} = {\sum\limits_{m = {1\mspace{14mu} \ldots \mspace{14mu} M}}{{A_{m}(\lambda)} \cdot x_{m}}}},$

wherein A_(m)(λ) is a fraction of said modification caused by the m^(th)actuator S_(m) upon receiving a unitary control signal by said actuator;

wherein the functions f₁ . . . f_(M) and F₁ . . . F_(N) are chosen so asto increase flatness of the gain profile G(λ).

In accordance with another aspect of the invention there is furtherprovided a method for flattening a gain profile G(λ) of a hybridamplifier comprising an erbium doped fiber amplifier and a Ramanamplifier for amplifying a stream of optical signals, the methodcomprising:

splitting a tapped portion of the stream of optical signals in the formof N+1 sub-portions;

measuring optical power value P₀ of the first said sub-portion;

spectral filtering remaining N sub-portions through N filters havingrespective transmission functions F₁(λ) . . . F_(N)(λ), and measuringoptical power values P₁ . . . P_(N) of the respective filteredsub-portions of the tapped portion;

generating M control signals x₁ . . . x_(M) based on the formulax_(m)=f_(m)(C_(k),P₀ . . . P_(N)), wherein f_(m) is a pre-determinedfunction and C_(k) are pre-determined constants, for each m=1 . . . M;

applying said M control signals x₁ . . . x_(M) to M spectral actuatorsS₁ . . . S_(M), respectively, wherein said actuators modify the gainprofile G(λ) by a value

${{\Delta \; {G(\lambda)}} = {\sum\limits_{m = {1\mspace{14mu} \ldots \mspace{14mu} M}}{{A_{m}(\lambda)} \cdot x_{m}}}},$

wherein

A_(m)(λ) is a fraction of said modification caused by the m^(th)actuator S_(m) upon receiving a unitary control signal by said actuator;

wherein the functions f₁ . . . f_(M) and F₁ . . . F_(N) are chosen so asto increase the flatness of the gain profile G(λ).

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings in which:

FIG. 1 is a configuration of a prior art apparatus for transient gaindeviation control in a hybrid erbium doped fiber amplifier-Ramanamplifier optical amplifier;

FIG. 2A-2C are general configurations of the apparatus of the presentinvention for flattening a gain profile of an optical amplifier;

FIG. 3 is an optical circuit of a preferred embodiment of the detectiondevice of the present invention;

FIG. 4 is an optical circuit of a preferred embodiment of the detectiondevice for the case of only a single filter and two detectors;

FIG. 5 is a calculated transmission filter function for the filter inthe detection device of FIG. 4;

FIG. 6 is a configuration of the apparatus of the present inventionshowing a particular implementation of the actuators;

FIG. 7 is a graph showing changes in overall amplifier gain due to Ramanamplifier and erbium doped fiber amplifier pump changes.

DETAILED DESCRIPTION OF THE INVENTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art.

Referring to FIG. 1, a prior art hybrid optical amplifier 800 is showncomprising a multiplexor 802, a first erbium doped fiber amplifier(EDFA) 808, a span of dispersion compensated optical fiber (DCF) 810, asecond EDFA 812, three Raman pump diode lasers 816 emitting pump lightat differing pump wavelengths, a Raman pump in-coupler 818 for couplingthe Raman pump to DCF 810, and a Raman pump out-coupler 820 for removinga residual Raman pump light 822. An incoming multi-wavelength signal824, carried by many optical fibers, is multiplexed by multiplexor 802to propagate in a single optical fiber 804 coupled to amplifier 808.After amplification by EDFAs 808 and 812 and by DCF 810 pumped withdiode lasers 816, the signal exits the amplifier as shown by an arrow826.

In order to correct dynamic gain tilt caused by variations in amountand, or optical power of signals at individual wavelengths comprisingincoming multi-wavelength signal 824, a compensation circuit isimplemented in the prior art amplifier 800 comprising three signalsources 828 at wavelengths λ₁, λ₂, and λ₃ coupled to multiplexor 802, anoutput tap 830, a demultiplexor 832 having outputs corresponding to thewavelengths λ₁, λ₂, and λ₃, which are coupled to three separatephotodetectors 834, and a controller 836 arranged to receive signalsfrom the photodetectors 834 and adjust drive currents of power supplies814 supplying the drive currents to three Raman pump diode lasers 816.

In operation, light at three wavelengths λ₁, λ₂, and λ₃ is used to probethe gain profile of amplifier 800 in real time. When a transient changeof the amplifier gain appears as a result of a change in the amplifierloading conditions, the ratio of optical power values of light at thesethree wavelengths changes which prompts the controller 836 to change theratio of drive currents of Raman pumps accordingly, so as to reducetransient effects and flatten the gain profile of amplifier 800.

FIG. 2A shows a preferred general configuration of the optical amplifierof the present invention with automatic control of the gain profile. Thesolid arrows represent optical signals and dashed arrows representelectrical or control signals. An optical amplifier 200A comprises a tap94, an EDFA 5, a gain adjuster 19, a detection device 11, and a controlunit 533. A small fraction of a multi-wavelength optical signal 201 istapped off by tap 94, while most of the signal proceeds to EDFA 5, whichamplifies the optical signal, and further to gain adjuster 19 whichadjusts the gain profile of the amplifier 200A so as to minimizedifferences between optical powers of signals at various wavelengthcomprising an output signal 202. The specific realization of gainadjuster 19 will be considered in more detail below. Detection device11, which will also be described in more detail below, produces a set ofelectrical signals to the control unit 533 which controls EDFA 5 andgain adjuster 19, so as to keep said differences between optical powersof signals at different wavelengths to a minimum.

All possible locations of gain adjuster 19, tap 94, and EDFA 5 will workwith respect to the present invention, but some configurations areeasier to implement than others. For example, in FIG. 2B, anotherpreferred configuration of the amplifier of the present invention isshown. In an amplifier 200B, gain adjuster 19 is located before EDFA 5,and tap 94 with detection device 11 is located after EDFA 5. Further, inFIG. 2C, an amplifier 200C is shown wherein gain adjuster 19 is insertedat a mid-stage of EDFA 5, and a part of the gain adjustment function iscarried by an EDFA itself. The role of gain adjuster device 19 can beperformed by a dynamic gain equalizer or by a Raman amplifier.

Turning now to FIG. 3, a detection device of the present invention isshown comprising a 1×(N+1) splitter 211, a photodetector 212, a set ofoptical filters 214-1 . . . 214-N, and a set of photodetectors 213-1 . .. 213-N. The first photodetector 212 measures optical power proportionalto the total power of the signal 201 coming through the tap 94. Theremainder of the photodetectors 213-1 . . . 213-N measure the opticalpower of the signal coming through splitter 211 and optical filters214-1 . . . 214-N, respectively. The transmission shapes F₁(λ) . . .F_(N)(λ) of these filters are selected in such a way that together withthe gain adjuster 19 of FIGS. 2A-2C they give optimum compensation ofthe EDFA spectral hole burning (SHB).

Once the powers P₁ . . . P_(N) at photodetectors 213-1 . . . 213-N aremeasured, the controller generates a vector of numbers x=x₁ . . . x_(M),where M is the amount of independently adjustable parameters of gainadjuster 19 in FIGS. 2A-2C. These parameters may correspond toindividual pump powers and, or variable optical attenuator (VOA)settings. The vector x is passed to gain adjuster 19 and to EDFA 5 ofFIGS. 2A-2C. It is assumed that the overall gain change due to thisadjustment, that is, the gain change between input 201 and output 202 ofFIGS. 2A-2C, is the following:

$\begin{matrix}{{{\Delta \; {G(\lambda)}} = {\sum\limits_{m = {1\mspace{14mu} \ldots \mspace{14mu} M}}{{A_{m}(\lambda)} \cdot x_{m}}}},} & (1)\end{matrix}$

where each A_(m)(λ) is the gain modification by a single “actuator”,that is, by the element of the gain adjuster 19 that is controlled byone of the component of the vector x. In other words, A_(m)(λ) is afraction of the gain modification caused by a m^(th) actuator uponreceiving a unitary control signal by said actuator. In equation (1),the gain modifications are expressed in dB units.

In the preferred embodiment the controller calculates vector x using thefollowing equation:

$\begin{matrix}{x_{m} = {C_{m,0} + {\sum\limits_{n = {1\mspace{14mu} \ldots \mspace{14mu} N}}{C_{m,n}\frac{P_{n}}{P_{0}}}}}} & (2)\end{matrix}$

Here, C_(m,n) are some constant coefficients obtained during systemdesign, P_(n) is the power measured at n-th detector in linear unitssuch as in milliwatt, and P₀ is the power measured at detector 212 ofFIG. 3, that is the total, or unfiltered, power. The total number ofdetectors is N+1.

Even though equation (2) gives very good results for SHB compensation,other formulas can be used for x_(i) calculation. The most genericformula is x_(m)=f_(m)(C_(k),P₀ . . . P_(N)), wherein f_(m) is apredetermined function and C_(k) are some predetermined constants.

During system design one needs to optimize the coefficients C_(m,n)together with filter shapes F₁(λ) . . . F_(N)(λ) in such a way that theoverall gain change is minimal for different loading conditions. Thiscan be done via simulation when optimization procedure runs throughrandomly generated signal loading conditions while adjustingcoefficients C_(m,n) and filter shapes F₁(λ) . . . F_(N)(λ). Upon eachadjustment, the optimization procedure calculates resulting gain changeand, out of all filter shapes and coefficients C_(m,n) tried, it choosesthe ones corresponding to the minimal perturbation of the original gainprofile. The calculated coefficients C_(m,n) are then stored in thememory of control unit 533 to generate vector x. Since coefficientsC_(m,n) are pre-calculated, the response time of the control unit 533can be in sub-microsecond domain which is fast enough to compensate formost transients caused by changes of loading conditions of amplifiers200A-200C of FIGS. 2A-2C.

The apparatus of present invention will work using different numbers ofdetectors and actuators. While increasing the number of detectors andactuators generally improves the degree of achieved gain profileflatness of amplifiers 200A-200C of FIGS. 2A-2C, an optimal number ofdetectors and actuators exists which is capable of adequatelycompensating for both SHB and stimulated Raman scattering tilt. Asimulation has shown that, surprisingly, only one filter, two detectors,and three or four actuators are sufficient to compensate for theseeffects.

In case of optimization involving more than one filter, the transmissionfunctions of the filters may have common regions of non-zerotransmission. Thus, the different filters are not just differentbandpass filters used to obtain optical powers in different areas of thespectrum of multi-wavelength optical signal to be amplified, as it is inthe case of, for example, an apparatus of U.S. Pat. No. 7,359,112.Advantageously, the spectral shapes F₁(λ) . . . F_(N)(λ) of the filtersof the present invention are optimized using the abovementionedoptimization procedure, so as to ensure that the filters 214-1 . . .214-N filter out signals which are most representative of transientperturbations of the amplifier gain profile caused by spectralvariations in optical signal 201 of FIG. 3.

Further, tap 94 and 1×(N+1) splitter 211 of the detection device of FIG.3 can be replaced by any combination of taps and splitters tapping aportion of signal 201 in the form of N+1 sub-portions, one sub-portionbeing coupled to detector 212 and remaining N sub-portions each beingcoupled to one of filters 214-1 . . . 214-N coupled to detectors 213-1 .. . 213-N, respectively. Any such modification would result in anoperational apparatus and, therefore, is a part of the presentinvention.

Turning now to FIG. 4, an optical circuit of a detection device is shownhaving a 1×2 splitter 2110, a filter 214, and two detectors 212 and 213.Similarly, tap 94 and 1×2 splitter 2110 of FIG. 4 can be replaced, forexample, by two taps, not shown, the first tap, not shown, being coupledto detector 212, and the second tap, not shown, being coupled to filter214 coupled to detector 213. Upon such modification, or any othersimilar modification, the apparatus will still perform its intendedfunction and, therefore, any such modification is a part of the presentinvention.

The filter transmission function F₁(λ) of filter 214 of FIG. 4, obtainedthrough the abovementioned optimization procedure, is shown in FIG. 5.The filter transmission function of FIG. 5 has a transmission peakreaching a maximum transmission at a wavelength of 1532±2 nm, anattenuation peak reaching a minimum transmission at a wavelength of1541±2 nm, and an intermediate transmission of between 10% and 30% ofthe maximum transmission minus minimum transmission within a 1550±5 nmwavelength band. An apparent drop below zero in FIG. 5 at 1541±1 nm is aresult of optimization, and, in a real filter, the transmission in thisregion can be taken equal to zero or, alternatively, the whole curve canbe shrunk to fit between 0% and 100% transmission. Both methods werefound to give adequate results.

Turning now to FIG. 6, another preferred embodiment of an amplifier 600of the present invention is shown comprising an EDFA 84 working togetherwith a distributed Raman amplifier comprising Raman pumps 53, a WDMcombiner 51 and a transmission fiber 500. The actuators are the Ramanpumps 53 and erbium doped fiber average inversion of EDFA 84. Theaverage inversion adjustment is performed by varying EDFA pump powers asis symbolically shown with an arrow 130 b. When EDFA pumps are adjusted,the average EDFA gain is measured via a detector 104 and detectors in adetection device 11 having two detectors and one filter, not shown.Detection device 11 receives an optical signal from a tap 91 locatedbefore EDFA 84, and passes corresponding electrical signals to a controlunit 535 through a line 113, and detector 104 receives a fraction of anoutput optical signal tapped by an output tap 92 and passescorresponding electrical signal to control unit 535 through a line 114.The measured gain is then held constant by control unit 535 viaadjustment of a VOA 85 through a line 135 b using(configuration-specific) values of C_(m,n) or C_(k) stored in itsmemory. Generally, VOA 85 can be positioned anywhere in amplifier 600,including before or after erbium doped fiber coils, not shown. Also,there can be more than one VOA, in this case any of the VOA or all ofthem can be adjusted. The pump powers of Raman pumps 53 are adjusted bycontrol unit 535 through a line 115.

An example of the actuator functions A_(m)(λ) is shown in FIG. 7.Functions 61 and 62 are the gain changes due to Raman pump change (twoRaman pumps in this example) and a function 63 is the change due toaverage inversion adjustments, before gain of EDFA 84 of FIG. 6 isadjusted by VOA 85 of same Figure. Using the average inversion actuatorreduces the need of having more than two Raman pump actuators for goodSHB compensation.

It should be noted that even though a distributed counter-propagationRaman amplifier topology is described in the preferred embodiment ofFIG. 6, the present invention is not limited to this particulartopology; other topologies can be used, such as co-pumping or discreteRaman amplifier located anywhere near or within EDFA.

Simulations over 520 randomly generated cases have shown that actuatorfunctions shown in FIG. 7 together with the filter function shown inFIG. 5 and with optimized coefficients C_(m,n) of equation (2) canreduce the gain change due to SHB by a factor of 2 on average. Forfurther reduction of the SHB induced changes one needs to increase thenumber of filters and detectors in detection device 11. The increase ofthe number of Raman pumps also helps with the SHB compensation but theimprovements are minor in case of a single filter 214, however theimprovements will be more significant together with larger number offilters.

1. An apparatus for flattening a gain profile G(λ) of a hybrid amplifiercomprising an erbium doped fiber amplifier (EDFA) and a Raman amplifier(RA) for amplifying a stream of optical signals, the apparatuscomprising: a detection device arranged to receive a tapped portion ofthe stream of optical signals in the form of N+1 sub-portions and toprovide N+1 output signals P₀ . . . P_(N) in dependence upon said N+1sub-portions, wherein N is an integer positive number, the detectiondevice comprising: N spectral filters having respective transmissionfunctions F₁(λ) . . . F_(N)(λ); and N+1 photodetectors for producing theN+1 output signals P₀ . . . P_(N), in response to a light impingingthereon, wherein the first sub-portion of the tapped portion of thestream of optical signals is coupled to the first photodetector forproducing the signal P₀, and each one of remaining N of saidsub-portions of the tapped portion of the stream of optical signals iscoupled to one of the N spectral filters coupled to one of the remainingN photodetectors for producing the signals P₁ . . . P_(N); a controllerarranged to receive said signals P₀ . . . P_(N) from the detectiondevice and suitably programmed to provide M control signals x₁ . . .x_(M) in dependence upon said signals P₀ . . . P_(N), wherein M is aninteger positive number and x_(m)=f_(m)(C_(k),P₀ . . . P_(N)), whereinf_(m) is a pre-determined function and C_(k) are pre-determinedconstants, for each m=1 . . . M; and M spectral actuators S₁ . . . S_(M)arranged to receive said control signals x₁ . . . x_(M), respectively,and modify the gain profile G(λ) by a value ΔG(λ) according to theequation${{\Delta \; {G(\lambda)}} = {\sum\limits_{m = {1\mspace{14mu} \ldots \mspace{14mu} M}}{{A_{m}(\lambda)} \cdot x_{m}}}},$wherein A_(m)(λ) is a fraction of said modification caused by the m^(th)actuator S_(m) upon receiving a unitary control signal by said actuator;wherein the functions f₁ . . . f_(M) and F₁ . . . F_(N) are selected soas to increase flatness of the gain profile G(λ).
 2. An apparatus ofclaim 1, wherein the set of functions f_(m) is defined as${f_{m} = {C_{m,0} + {\sum\limits_{n = {1\mspace{14mu} \ldots \mspace{14mu} N}}{C_{m,n}\frac{P_{n}}{P_{0}}}}}},$where C_(m,n) are M·N pre-determined constants.
 3. An apparatus of claim1, wherein N>1 and wherein the transmission functions F₁ . . . F_(N)have at least one overlapping wavelength region having substantiallynon-zero transmission.
 4. An apparatus of claim 1, wherein the detectiondevice further comprises a 1×(N+1) splitter for splitting the tappedportion of the stream of optical signals into the N+1 sub-portions. 5.An apparatus of claim 1, wherein the M spectral actuators are selectedfrom a group consisting of a pump of the EDFA, a pump of the RA, and avariable optical attenuator (VOA).
 6. An apparatus of claim 1 furthercomprising a second separate RA, wherein the M spectral actuators areselected from a group consisting of a pump of the EDFA, a pump of theRA, a pump of the second separate RA, and a variable optical attenuator(VOA).
 7. An apparatus of claim 1, wherein the tapped portion of thestream of optical signals is tapped at a location upstream of the hybridamplifier.
 8. An apparatus of claim 1, wherein the tapped portion of thestream of optical signals is tapped at a location downstream of thehybrid amplifier.
 9. An apparatus of claim 1, wherein the tapped portionof the stream of optical signals is tapped at a location inside thehybrid amplifier.
 10. An apparatus of claim 1, wherein N=1 and thetransmission function F₁(λ) has a transmission peak reaching a maximumtransmission at a wavelength of 1532±2 nm, an attenuation peak reachinga minimum transmission at a wavelength of 1541±2 nm, and an intermediatetransmission of between 10% and 30% of the maximum transmission minusminimum transmission, wherein said intermediate transmission is achievedwithin a 1550±5 nm wavelength band.
 11. An apparatus of claim 5, whereinthe pump of the RA propagates towards the stream of optical signals. 12.An apparatus of claim 5, wherein M=3.
 13. An apparatus of claim 5,wherein M=4.
 14. An apparatus of claim 12, wherein the actuatorscomprise one Raman pump, one EDFA pump, and one VOA.
 15. An apparatus ofclaim 13, wherein the actuators comprise two Raman pumps, one EDFA pump,and one VOA.
 16. A method for flattening a gain profile G(λ) of a hybridamplifier comprising an erbium doped fiber amplifier (EDFA) and a Ramanamplifier (RA) for amplifying a stream of optical signals, the methodcomprising: splitting a tapped portion of the stream of optical signalsin the form of N+1 sub-portions; measuring optical power value P₀ of thefirst said sub-portion; spectral filtering remaining N sub-portionsthrough N filters having respective transmission functions F₁(λ) . . .F_(N)(λ), and measuring optical power values P₁ . . . P_(N) of therespective filtered sub-portions of the tapped portion; generating Mcontrol signals x₁ . . . x_(M) based on the formula x_(m)=f_(m)(C_(k),P₀. . . P_(N)), wherein f_(m) is a pre-determined function and C_(k) arepre-determined constants, for each m=1 . . . M; applying said M controlsignals x₁ . . . x_(M) to M spectral actuators S₁ . . . S_(M),respectively, wherein said actuators modify the gain profile G(λ) by avalue${{\Delta \; {G(\lambda)}} = {\sum\limits_{m = {1\mspace{14mu} \ldots \mspace{14mu} M}}{{A_{m}(\lambda)} \cdot x_{m}}}},$wherein A_(m)(λ) is a fraction of said modification caused by the m^(th)actuator S_(m) upon receiving a unitary control signal by said actuator;wherein the functions f₁ . . . f_(M) and F₁ . . . F_(N) are chosen so asto increase the flatness of the gain profile G(λ).
 17. A method of claim16, wherein the set of functions f_(m) is defined as${f_{m} = {C_{m,0} + {\sum\limits_{n = {1\mspace{14mu} \ldots \mspace{14mu} N}}{C_{m,n}\frac{P_{n}}{P_{0}}}}}},$where C_(m,n) are M·N pre-determined constants.
 18. A method of claim16, wherein N>1 and wherein the transmission functions F₁ . . . F_(N)have at least one overlapping wavelength region having substantiallynon-zero transmission.
 19. A method of claim 16, wherein N=1, M=3.
 20. Amethod of claim 16, wherein N=1, M=4.